The liquid crystal display has become the mainstream of flat plate display, and active driven TFT-LCD has become the mainstream among the display manners of LCDs. The manufacturing process of TFT-LCD is compatible with that for the conventional integrated circuits. In addition, the TFT-LCD has the advantages of excellent display quality, low power consumption, light weight, and low radiation, and is a friendly man-machine interactive interface. The applications of TFT-LCD comprise notebook computer, desktop computer, workstation, industrial monitor, global satellite positioning system (GPS), personal digital assistant (PDA), game machine, visual telephone, portable VCD and DVD, and other portable equipment.
In order to effectively reduce the cost of TFT-LCD and improve the yield, efforts have been made to simplify the manufacturing process of the TFT-LCD array substrate. The array substrate was fabricated by a seven-mask or six-mask (7Mask or 6Mask) process in the beginning and generally by a five-mask (5Mask) process currently. Recently, a four-mask (4Mask) process using a gray tone mask has been applied to manufacture the TFT-LCD array substrate and is becoming widespread. An important step in the 4Mask process is to perform the photolithography for the active layer (Active Mask) and the photolithography for the source/drain electrode (S/D Mask) in the conventional 5Mask process with one gray tone mask.
The 4Mask process in the related art is briefly described as follows. First, a gate metal layer is deposited on a substrate, and a gate line and a gate electrode are formed with the first photolithography. Secondly, a gate insulating layer, an active layer, an ohmic contact layer, and a source/drain metal layer arc deposited successively on the gate line and the gate electrode. With the second photolithography, a data line, an active region, a source/drain electrode, and TFT channel are formed by source/drain wet etching and multiple-step etching (active layer etching→ashing→Mo dry etching→n+etching). Thirdly, a passivation layer is deposited, and a via hole is formed in the passivation layer with the third photolithography. Lastly, a transparent pixel electrode layer is deposited, and a pixel electrode is formed with the fourth photolithography.
Although the above 4Mask process is advantageous over the conventional 5Mask process, the 4Mask process still has some drawbacks. For example, the multiple-step etching is complicated and difficult to perform, and will inevitably bring about some defects, such as the residue of metal Mo, rough surface of the channel, etc. In addition, the lateral etching during the Mo dry etching will affect the aspect ratio of the channel and cause the change of electric characteristics of the TFT, such as low on-state current.